An Unusual Virus

“One striking feature of Ebola is that, unlike most viruses, it replicates in a lot of tissues,” says Vincent Racaniello, a Columbia professor of microbiology and immunology who runs Virology Blog. “You can take the virus in through your mouth, nose, eyes, or skin, and it can spread throughout your body and grow in many different types of cells — in your respiratory tract, gut tract, skin, muscle, liver. That’s unusual for viruses. Usually, they’re restricted to just a few places, like influenza [respiratory tract] or polio [intestinal tract and nervous system]. We call that tropism: the cells and tissues where the virus replicates. Ebola is interesting because its tropism doesn’t seem to be regulated in any way.”

Like many scientists, Racaniello thinks that Ebola’s natural host is a fruit bat.

“All the viruses that we recognize in humans originated in animals,” says Racaniello, who in 1981 was part of the team at MIT that sequenced the poliovirus genome. “Influenza originates in birds, for example. Polio originated in animals, but now it’s strictly a human infection: it goes from person to person. But Ebola is not a human virus: every outbreak starts with the virus going from an animal to a human, and then it goes human-human-human, until we’re able to stop the transmission. The virus is then gone. When another outbreak happens, it’s from another animal entry,” probably from touching or eating an infected animal.

Once inside the human body, the virus works by entering a cell and then taking over the cell’s machinery to reproduce itself, making more Ebola proteins, which then break out of the cell and infect other cells.

“This is an RNA virus,” explains Columbia epidemiologist Stephen Morse. “Many viruses — Ebola, influenza, polio — have RNA genomes, and they all need their own special enzymes to copy themselves, because our body doesn’t know what to do with them. That’s why there are so many mutations in RNA viruses — those copying mechanisms are very sloppy. We don’t normally copy over RNA in our own bodies. DNA viruses like herpes simply use the DNA-copying machinery of our own cells.” Despite the profuse mutations caused by the RNA-copying process, neither Morse nor Racaniello see any real chance of the virus becoming airborne, as some people fear. “We have been studying viruses for over a hundred years,” Racaniello says, “and we’ve never seen a virus in humans change the way it is transmitted.”

As for the ghastly hemorrhaging — a result of damage to the cells that line the blood vessels — Morse says, “About 40 percent of cases get these dramatic Hollywood effects. So if you’re looking for that, very often you’ll miss the diagnosis.”